1. Field of the Invention
The present invention relates to a motor control device which prevents overheating of a motor coil, a walking assist device equipped with the motor control device, and a motor control method.
2. Description of the Related Art
In a motor provided in a walking assist device or the like, a coil is maintained in an energized state in order to keep a rotation angle not only during rotation, but also during stop of rotation. The coil is heated along with energization. If a heating value of the coil exceeds a heat discharge value thereof, the temperature of the coil rises, thereby placing the coil in an overheated state. Therefore, it is necessary to protect the coil not to reach the overheating temperature during use of the motor.
Patent Document 1 (Japanese Patent Application Laid-Open No. 2002-238293) discloses that motor current is limited on the basis of an integrated value of a predetermined function of a phase current or an integrated value of a power function of a phase current as a countermeasure against overheating of a three-phase motor (paragraph 0047 in Patent Document 1). Patent Document 1 also discloses that, in the three-phase motor, an integrated value of a phase current is calculated for each phase to limit all phase currents on the basis of the integrated value in a phase having the maximum integrated value (paragraph 0036 in Patent Document 1).
Patent Document 2 (Japanese Patent Application Laid-Open No. 2007-282478) discloses that, in a brushless DC motor, calculation is made to obtain an integrated value of a difference between an integrated value of a phase current or a function value of a predetermined function of a phase current and a threshold value, the temperature of a permanent magnet is subsequently predicted on the basis of the integrated value and an intermediate bus voltage of an inverter, and then the maximum current of the motor is limited on the basis of the predicted temperature (paragraph 0037 in Patent Document 2).
In the conventional countermeasures against overheating of a motor, a phase current is limited according to the present phase current, the present integrated value obtained by integration of the phase current, or the present values of other functions, and therefore the phase current is not limited on the basis of whether the coil temperature obtained after the lapse of the predetermined time from the present reaches the overheating temperature. In the conventional countermeasures against overheating of the motor, in order to deal with a time delay which occurs until the coil temperature actually decreases or an overshoot of the coil temperature after starting control of decreasing the phase current, the threshold value is set to a value corresponding to a temperature sufficiently lower than a guaranteed upper limit temperature of the coil to control the present various values so as to be lower than the threshold. Therefore, in the conventional countermeasures against overheating of the motor, the setting of the threshold value corresponding to the lower temperature suppresses the coil current earlier though the coil current is still able to be increased, by which the motor performance is not fully used.
Therefore, it is an object of the present invention to provide a motor control device, a walking assist device equipped with the motor control device, and a motor control method capable of setting the upper limit of a coil temperature to a value sufficiently close to a guaranteed upper limit temperature while securing that the coil temperature does not exceed the upper limit, thereby suppressing the limitation on the coil current.